• Skip to primary navigation
  • Skip to main content
  • Skip to primary sidebar
  • Skip to footer

Analog IC Tips

Analog IC Design, Products, Tools Layout

  • Products
    • Amplifiers
    • Clocks & Timing
    • Data Converters
    • EMI/RFI
    • Interface & Isolation
    • MEMS & Sensors
  • Applications
    • Audio
    • Automotive/Transportation
    • Industrial
    • IoT
    • Medical
    • Telecommunications
    • Wireless
  • Learn
    • eBooks / Tech Tips
    • FAQs
    • EE Learning Center
    • EE Training Days
    • Tech Toolboxes
    • Webinars & Digital Events
  • Resources
    • Design Guide Library
    • Digital Issues
    • Engineering Diversity & Inclusion
    • LEAP Awards
    • Podcasts
    • White Papers
    • DesignFast
  • Video
    • EE Videos
    • Teardown Videos
  • EE Forums
    • EDABoard.com
    • Electro-Tech-Online.com
  • Engineering Training Days
  • Advertise
  • Subscribe

The PC-board atomic clock, Part 1: Primary and secondary standards

September 23, 2021 By Bill Schweber

Atomic clocks have shrunk from room-sized assemblies to PC-board mountable modules, admittedly with lower performance than their big siblings but still far better than crystal-based timing alternatives.

Atomic clocks are based on the transitions of energy states of specific elements such as cesium, ytterbium, rubidium, and others under specific and tightly controlled conditions. They are the absolute standard of time in meteorology. In the United States, the National Institute for Standards and Technology (NIST) developed, operates, and maintains NIST-F1, the nation’s primary time and frequency standard located in Boulder, Colorado. This cesium “fountain” atomic clock contributes to the international group of atomic clocks that define Coordinated Universal Time (UTC), the official world time.

In the SI (Système international- International System of Units) definition of base units, the second is defined by setting the fixed numerical value of the cesium frequency ΔνCs (the unperturbed ground-state hyperfine transition frequency of the cesium-133 atom) to be 9,192,631,770 when expressed in the unit Hertz (Hz, the reciprocal of seconds). Primary-standard atomic clocks require supercooled plumbing near absolute zero, lasers, optics, and electronics, as well as considerable operating power and attention, and occupy a large room (Figure 1).

Fig 1: This is the core of the National Institute for Standards and Technology NIST-F1 primary standard atomic clock; the extensive support systems are not shown. (Image: NIST)

Obviously, the NIST primary-standard arrangement isn’t practical as a component or system on a test bench or even a satellite in space. Nearly all satellites, whether in Earth orbit or deep space, carry an atomic clock; in fact, every satellite in the GPS constellation has one plus a backup. The availability of such clocks is key to the accuracy of the GPS-defined location functionality. The atomic clocks used in satellites are often customized versions of the secondary-standard atomic clocks that you can buy as a standard item from various instrumentation vendors.

For example, atomic clocks such as commercial rubidium frequency standards operate by phase-locking (called “disciplining”) a crystal oscillator to the hyperfine transition at 6.834,682,612 GHz in rubidium. The amount of light from a rubidium discharge lamp that reaches a photodetector through a resonance cell will drop by about 0.1 % when the rubidium vapor in the resonance cell is exposed to microwave power near the transition frequency. The crystal oscillator is stabilized to the rubidium transition by detecting the light dip while sweeping an RF frequency synthesizer (referenced to the crystal) through the transition frequency. (Atomic clocks based on cesium or other elements have some similarities in operating principles and significant differences – all too complicated to explore here.)

These secondary standards are used in satellites, Earth stations, major cellular and Internet backbones nodes, and even high-end test and measurement systems, with the 19-inch rack model being very common. While these rack-mount units have an accuracy which is many orders of magnitude than primary units such as NIST-F1, they are still far better, again by many orders of magnitude, than even the best crystal-controlled clock (oscillator), including high-end oven-controlled (OXCO) or medium-to-high performance temperature controlled (TXCO) units.

There are even smaller units, such as the Stanford Research Systems PRS-10, a rubidium-based atomic clock. It measures just 2.00″ × 3.00″ × 4.00″, weighs 1.32 pounds, requires +24 VDC/0.6 A in steady-state operation (after 2.2 A warm-up), and sells for about $2000, even though the complete unit requires a considerable amount of support circuitry (Figure 2).

Fig 2: It takes a lot more than an atomic “physics package” for an atomic clock, but modern electronics makes this very practical and compact. (Image: the Stanford Research Systems)

The physical construction of such a unit with its circuit boards around the rubidium core (Figure 3 and Figure 4) is critical to achieving compact packaging and reaching the highest level of potential performance, as thermal stability and uniformity play a large role.

Fig 3: At the heart of the Stanford Research Systems PRS10 is a rubidium oscillator. (Image: the Stanford Research Systems)
Fig 4: The oscillator of the PRS10 is surrounded by the required electronics for compact packaging and enhanced thermal stability and uniformity. (Image: the Stanford Research Systems)

The next section will look at the development, design, and overall operation of the Chip-Scale Atomic Clock (CSAC), a standard commercial module from Microchip Technology which can be mounted on a PC board.

EE World Related Content

GPS is a ubiquitous and problematic technology
Basics of GPS receivers
GPS, Part 1: Basic principles
GPS, Part 2: Implementation
Atomic clock works from 5 V, outputs CMOS-compatible signals
Chip-scale atomic clock produces super-stable frequencies
Miniaturized rubidium atomic clock maintains high degree of synchronization to reference clock
Commercial space Chip Scale Atomic Clock module is radiation tolerant
Chip-scale atomic clocks fit in compact quarters
Quartz crystals and oscillators, Part 1: Crystal basics
Quartz crystals and oscillators, Part 2: Advanced crystals

References

  • National Institute for Standards and Technology, “NIST-F1 Cesium Fountain Atomic Clock”
  • National Institute for Standards and Technology, “Definitions of SI Base Units”
  • National Institute for Standards and Technology, “Analysis of Time Domain Data” (Allan Deviation).
  • Stanford Research Systems, “Rubidium Oscillator PRS10”
  • Stanford Research Systems, “Frequency Standards: PRS10 — Rubidium frequency standard with low phase noise”
  • Symmetricom, “The SA.45S Chip-Scale Atomic Clock”
  • Wikipedia, “Allan variance.”
  • Microchip Technology, CSAC SA65 Data Sheet
  • Microchip Technology, “Chip-Scale Atomic Clock (CSAC) SA65 User’s Guide”
  • Microchip Technology, Robert Lutwak, “Tactical Atomic Clocks”
  • Lutwak, et al; 36th Annual Precise Time and Time Interval (PTTI) Meeting, “The Chip-Scale Atomic Clock – Low-Power Physics Package” (2004)
  • Mescher, et al, “An Ultra-Low-Power Physics Package For a Chip-Scale Atomic Clock”
  • 2011 Stanford PNT Symposium, Robert Lutwak, “The SA.45S Chip-Scale Atomic Clock”
  • US Patent 6,927,636 B2 (2005), “Light stabilization for an optically excitable atomic medium”
  • US Patent 6,320,472 B1 (2001), “Atomic Frequency Standard”
  • US Patent 7,215,213 B2 (2007), “Apparatus and system for suspending a chip-scale device and related methods”

 

You may also like:

  • atomic clock
    The PC-board atomic clock, Part 3: The CSAC cesium physics…
  • atomic clocks
    The PC-board atomic clock, Part 2: The CSAC design

  • Quartz crystals and oscillators, Part 1: Crystal basics
  • GPS
    GPS, Part 1: Basic principles

  • Clock Tree 101

Filed Under: Clocks & Timing, FAQ, Featured, Products Tagged With: FAQ

Primary Sidebar

Featured Contributions

Design a circuit for ultra-low power sensor applications

Active baluns bridge the microwave and digital worlds

Managing design complexity and global collaboration with IP-centric design

PCB design best practices for ECAD/MCAD collaboration

Open RAN networks pass the time

More Featured Contributions

EE TECH TOOLBOX

“ee
Tech Toolbox: 5G Technology
This Tech Toolbox covers the basics of 5G technology plus a story about how engineers designed and built a prototype DSL router mostly from old cellphone parts. Download this first 5G/wired/wireless communications Tech Toolbox to learn more!

EE LEARNING CENTER

EE Learning Center
“analog
EXPAND YOUR KNOWLEDGE AND STAY CONNECTED
Get the latest info on technologies, tools and strategies for EE professionals.

EE ENGINEERING TRAINING DAYS

engineering

RSS Current EDABoard.com discussions

  • dc-dc converter in series
  • Right Half Plane Zero
  • Single ended measuring ports and balanced antenna
  • Thermal modelling of repetitive power pulse
  • Permittivity and Permealibility in CST

RSS Current Electro-Tech-Online.com Discussions

  • Can I make two inputs from one??
  • Is AI making embedded software developers more productive?
  • Kawai KDP 80 Electronic Piano Dead
  • Fun with AI and swordfish basic
  • Simple LED Analog Clock Idea
“bills

Design Fast

Component Selection Made Simple.

Try it Today
design fast globle

Footer

Analog IC Tips

EE WORLD ONLINE NETWORK

  • 5G Technology World
  • EE World Online
  • Engineers Garage
  • Battery Power Tips
  • Connector Tips
  • DesignFast
  • EDA Board Forums
  • Electro Tech Online Forums
  • EV Engineering
  • Microcontroller Tips
  • Power Electronic Tips
  • Sensor Tips
  • Test and Measurement Tips

ANALOG IC TIPS

  • Subscribe to our newsletter
  • Advertise with us
  • Contact us
  • About us

Copyright © 2025 · WTWH Media LLC and its licensors. All rights reserved.
The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of WTWH Media.

Privacy Policy